BRUSSELS, March 20 (Reuters) - The European Union said on Tuesday that the right of citizens from poorer member states to work in richer ones on a low salary would be limited to 18 months under a reform of the bloc's labour laws sought by France.

The new law, promoted by French President Emmanuel Macron and backed by Luxembourg, Belgium and the Netherlands, among others, would rein in current rules on the so-called posting of workers, which richer EU states say undercut their labour markets.

Poorer EU states from Spain to Poland have opposed the change, saying their citizens should be allowed to work in a wealthier state on a lower salary than a worker from the host country under the bloc's competition rules.

The deal, which had been tentatively agreed earlier this month, also introduces a two-year transition period. It is likely to be finally endorsed in April in a session in which Poland and Hungary expect to be outvoted.

Under the incoming rules, posted workers would start earning the host country salary after the maximum period allowed.

The European Parliament on Tuesday hailed the deal as a way to ensure "equal pay for equal work" but Poland's Deputy Foreign Minister Konrad Szymanski said it was a case of more powerful EU states like France imposing their will on the others.

"Such initiatives undermine the European project because they undermine its fundamental elements - the single market, the freedom to provide services, the freedom of movement for workers," he told reporters.

"Unfortunately, member states have not gathered enough resolve to tame such ideas."

An estimated 2 million posted workers in the EU currently make up only a tiny fraction of the bloc's workforce, but the issue has become politically highly sensitive. (Reporting by Gabriela Baczynska; Editing by Susan Fenton)